The United States Public Switched Telephone Network (PSTN) includes a number of different components. Each of these components was originally under the ownership and control of a single telephone service provider. Currently, however, respective components in the PSTN may be owned and/or under the responsibility of different parties. For example, a long distance telephone company may be responsible for long distance lines, whereas a local telephone company may be responsible for local lines, and a subscriber may be responsible for equipment at the subscriber premises.
When a malfunction in the PSTN arises, it is necessary to identify whether the malfunction is due to a problem located at the subscriber premises, in the subscriber loop or in the network of a chosen service provider. Responsibility for correcting the problem lies with the party responsible for the component where the problem lies. Matters are complicated by multiple service providers competing for local telephone communication services. As a result, the subscriber loop may be owned and maintained by one company yet leased by another company that provides local services to subscribers. Hence, each local service provider needs a way to immediately test for proper functioning of the network as far as a network interface device (NID). The NID is positioned at the DEMARC, i.e., the demarcation point where a local telephone company responsibility stops and the subscriber responsibility begins. Typically, the NID is located either adjacent to a subscriber's premises or a short distance away from the subscriber's premises. Thus, there is a need to be able to determine the location of a problem within the PSTN without incurring significant overhead and without suffering substantial delay.